A typical application field for the invented mask is the manufacturing of wafers with micromechanical devices, e.g. cantilevers for AFMs, pressure- and gas-sensors. These devices are often made of very thin Si-membranes whereas surrounding areas of the wafer remain at the original wafer thickness. The membranes are usually created by thinning the wafers of the backside. Further processing of the membranes from the backside is difficult or even impossible, e.g. when using spin coating of photoresists over such large topographies, since very large thickness variations of the coated film occur and the extreme required depth of focus limits the achievable resolution. Therefore, membrane-type devices are usually processed from the top side of the membrane while the wafer has its original thickness. This however limits the achievable functionality and fabrication simplicity of the devices.
U.S. Pat. No. 5,234,781 describes a mask for lithographic patterning and a method of manufacturing the same. Therein a substrate is to be processed from both sides, the front side and the back side. Therewith, a substrate is obtained with a recess which has a very big depth compared to the original substrate thickness. The recess hence leaves a membrane-like region in the substrate. To obtain a patterning of the membrane-like region, the substrate is processed from the front side before the recess is created from the back side.
Another example of that kind is provided in U.S. Pat. No. 4,855,197 wherein a mask for ion, electron or x-ray lithography and a method of making it is described.
Since the patterning of the membrane in the state of the art is conducted from the front side, fairly good results with conventional mask-assisted lithography methods are achieved.
A conventional method using conventional masks would lead to unacceptable patterning results when being used for a back side lithography of an already existing membrane-like region. This results e.g. from the so-called shadow effect which can create a parallax of the created pattern towards the wanted pattern. Another effect is the blurring of the mask pattern due to beam widening in the space between the mask and the surface.